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Modifiers: Vertex Weight Edit add invert curve falloff option

This commit adds the option to invert the resulting weights of the
falloff curve.

There is a workflow used by some to convert a texture mask into
vertex weights by using a custom curve and inverting the points.
This allows the same effect with a single click, and gives the modifier
more procedural functionality.

With minor UI tweaks by @mont29.

Differential Revision: https://developer.blender.org/D6899